Field of the Invention
Under favorable conditions of radio propagation where a signal does not undergo prohibitive distortions, using modulation superimposed on normally modulated signals is known. These favorable transmission conditions include high frequency line-of-sight, medium frequency groundwave, and low and very low frequency skywave. Phase and frequency modulations superimposed on normal modulations such as frequency shift keying (FSK), phase shift keying (PSK), on-off keying (OOK), other types of binary, M-ary and CW signals may be used: a. to increase the commumication capacity; b. to conceal a message; c. to synchronize bits and characters.
In addition, time modulation could be superimposed, as in a variation of the element lengths used in the OOK signalling. Time modulation consists of varying the length of a baud or a bit. Such time modulation could be superimposed on any other modulation to increase the bit rate or conceal the time modulation by changing the length of the bit by a slight amount - i.e., several different symbols would be generated by several different changes in bit length.
U.S. Pat. No. 2,692,330 issued on Oct. 19, 1954, to L. R. Kahn discloses a noise reduction system wherein phase modulation (PM) signals are superimposed on frequency shift keyed (FSK) signals. See also U.S. Pat. No. 3,916,313 issued Oct. 28, 1975, to R. B. Lowry, disclosing a PSK/FSK spread spectrum modulation/demodulation system.
It can be shown that the a particular complex signal spectrum can be generated by any one of a plurality of modulation types. However, the manner of detecting, demodulating, or analyzing such a complex signal spectrum without a priori knowledge of the modulation has heretofore not been known.
In general, certain known systems are used to detect the usual FSK, PSK, OOK, other types of binary and CW signals but they are not capable of recognizing the presence of additional modulation, let along decode it. In particular, if the frequency or phase modulation is imposed at a very slow rate with a small total deviation, the modulation will be undetectable in the existing systems. Depending upon the superimposed modulation used, it might be necessary to detect phase changes as small as one degree and frequency changes as low as 0.1 Hz. or perhaps even smaller in the presence of the normal frequency, phase, ON-OFF modulation and apparent CW signals.